1. Field of the Invention
This invention relates to system of maintaining the inventory of articles or objects provided with radio frequency (RF) transducers such as tags or transponders containing electronic codes for recognition and identification of the articles. Such devices are commonly known as radio frequency identification devices (RFID). More specifically, this invention relates to methods of employing radio frequency for spatial resolution of tags, RFID tags and tags activation devices. A RFID consists of a reader-interrogator and a plurality of transponders; and the latter are affixed on the objects or articles which are subject to inventory and may be located in a storage such as a warehouse.
2. Background Art
RFID methods and systems provide the recognition of objects with identification tags affixed thereon. The process of tag recognition must be accomplished at high speed and with minimum error. In the process, it is necessary to determine the Electronic Product Code (EPC) that describes the article to which the tag is attached, and the tag location or direction relative to a reader. Some of the interrogators are provided for primarily reading the tag codes while others are only for searching for the directions of the tags. An interrogator transmits a tag activation signal for all the tags in a predetermined interrogation zone simultaneously. It adjusts the activation signal which has been sent in advance to the tags with known ID or without ID codes depending on the tag design. If the tag ID is known in advance, it will be activated accordingly such that the interrogator can read its tag electronic code with high level of accuracy because there are no other response signals from other tags. When a small number of tags, for example, one to five tags, without ID codes are activated, because of the differences in electronic circuit parameters, the tags are activated in an insignificant time lag. Furthermore, the interrogator may activate the tags repeatedly so as to increase the probability of accurate recognition of the codes. However, when a large number of tags are to be read by the reader, the response tag signals reach the reader practically simultaneously which may result in failure to recognize the objects with adequate accuracy even in the case of tag signal processing with some of the anti-collision protocols. Miscellaneous tri-angular methods and reader multi-antenna design have been employed for resolving the above problem.
The RFID Handbook by Klaus Finkenzeller, Carl Hansen Verlag, Munich/FRG, 1999 outlines four methods of solving the problem of space, frequency, code and time discriminations in RFID.
U.S. Pat. Nos. 6,600,443 and 6,476,756 both to J. A. Landt, and U.S. Pat. No. 6,069,564 to R. Hatano et al illustrate methods and systems tag reading and the determination of its direction. The Landt patents illustrate a method of tag signal structure analysis while the Hantano et al patent proposes a multi-directional RFID antenna for this purpose.
Canadian Patent No.2,447,975 to P. M. Eisenberg et al, and No.2,399,092 and No.2,450,189 both to P. A. Sevcik et al describe aspects of the collection and use of data obtained by RFID tag interrogation, in particular, by comparing information obtained through interrogation of tags with the data recorded during repeated interrogation.
U.S. Pat. No. 6,317,028 to c. Valinlis; U.S. Pat. No. 5,822,714 to R. T. Cato; U.S. Pat. No. 6,034,603 to W. E. Steeves; and Canadian Patent No.2,447,975 to P. M. Eisenberg et al show RFID systems of tag recogniation for the case of a plurality of radio frequency identification tags. To effectively recognize tags, a number of other technical solutions assume a tag data base as previously known and perform its current status control through comparison of the read current values with the data of a base as shown in U.S. Pat. No. 5,822,714 to R. T. Cato.
U.S. Pat. No. 6,034,603 to W. E. Steeves also shows a method and system of tag construction with improved tag interference avoidance in which a tag includes both a receiver module and a processor, while the generation of a signal is decided as a result of analysis of radio frequency activity.
U.S. Pat. No. 7,030,761 to R Bridgelall et al shows a multi-resolution object location system and method for locating objects which employs a long range object locator together with a more precise RFID locator. The long range locator is used to first determine the general location of the object, and then the RFID locator further determines a more accurate location of the object.
U.S. Pat. No. 7,038,573 to G. Bann shows systems and methods for tracking the location of items within a controlled area having a plurality of RFID tags. Vehicles configured to transport the items being tracked are provided with two RFID interrogators to obtain the location of the vehicle.
U.S. Pat. No. 7,042,358 to S. E. Moore shows a method and apparatus for tracking items automatically in which a passive tag is used with remote sensing antennas placed at each remote location and a host computer communicates with the interrogators to determine item locations to an exacting measure.
U.S. Pat. No. 7,046,145 to W. Maloney shows RFID an object tracking and control system having a storage receptacle with a tray provided with an array of slots for receiving ID tags bearing touch memory devices. A computer-based controller detects the absence or presence and identity of ID tags disposed in the slots.
None of the above patents teach any RFID method and system possessing features which can perform recognition and locating functions of a plurality of objects as well as reading the codes and locating tags of both single decoding or working simultaneously with large numbers of articles under conditions of locating the inventory objects on a plane or in a random volume with minimization of errors caused by the reflection of signals form surrounding surfaces. Furthermore, the prior art patents fail to suggest, any RFID method of tag recognition and location in an interrogator close zone—Fresnel Zone when the distance between the tag and the reader antenna is relatively small and comparable with the antenna aperture.